Process and plant for preheating a metal charge fed in continuous to an electric melting furnace

ABSTRACT

A process and a plant for preheating a metal charge fed in continuous to an electric melting furnace through a preheating tunnel provided with a horizontal conveyor, wherein the metal charge is hit, in countercurrent, by the exhaust fumes or gas leaving the electric melting furnace and by jets of gas ejected through a plurality of nozzles positioned on the hood of the tunnel. The nozzles are arranged in groups interspaced from each other in a longitudinal direction with respect to the tunnel, and generate a small-scale turbulence or inject small fast gas jets that can penetrate the main gas stream passing through the preheating tunnel, and simultaneously generate a “horseshoe vortex” structure composed of a descending central gas flow (“downwash”), and ascending flows (“upwash”) close to the side walls of the preheating tunnel, which enable a desired circulation of the gases.

FIELD OF THE INVENTION

The present invention relates to an improved process for preheating ametal charge material fed in continuous to an electric furnace for theproduction of molten metal.

The invention also relates to a plant for implementing this process.

BACKGROUND OF THE INVENTION

A process and plant for pre-heating a metal charge (generally scrap) iswell known to skilled persons in the field, wherein said charge is fedin continuous to an electric melting furnace by means of a horizontalconveyor; said preheating process facilitates the subsequent meltingprocess.

The pre-heating of the charge takes place during the passage inside atunnel in which the sensible heat and combustion heat of the exhaustgases of the melting process are exploited (in some particular casespre-heating could also be favored by suitable auxiliary burners). Theexhaust fumes are then evacuated from the preheating tunnel and sent toa suitable treatment system. The combustion heat that is exploited inthe preheating process is essentially provided by the completion of thecombustion of the CO (carbon monoxide) and H.sub.2 (hydrogen) releasedby the process underway in the melting furnace, whereas the necessaryoxygen is generally provided with the supply of environmental air.

A process and a plant such as those briefly described above aredisclosed for example by U.S. Pat. No. 5,400,358, which describes theinjection of the air needed for combustion uniformly along thepreheating tunnel. This solution teaches the injection of airdistributed along the preheating tunnel in such a quantity as toguarantee an excess of oxygen in the order of 3-5% so as to ensure thecomplete combustion of the unburnt gases, assuming that the seal betweenfurnace and tunnel is almost perfect. In plant-engineering practice, ithas been seen that this situation cannot be achieved and there arealways significant infiltrations of external air (in particular at theinterface between the furnace and tunnel), often to an extent alreadymore than sufficient for ensuring the complete combustion of the processgases exiting from the furnace; it has also been observed that these airinfiltrations are not able to reach sufficient turbulence conditions, asthis air tends to follow the internal walls of the heating tunnel, andmixing and combustion with the process gases take place slowly. Anexcessive infiltration of ambient air into the preheating tunnel must beabsolutely avoided as it would overly lower the temperature of thegases, and if this temperature reaches a value very close to or evenlower than the CO ignition limit, there is the risk of not completingits combustion with the consequent release of this toxic gas into theenvironment, in addition to the significant loss of efficiency of thepre-heating system of the charge.

Within the context of the technical solution described above, theinjectable air from the hood is consequently extremely low if not zero;this fact worsens the problem related to the low turbulence, andprevents the best exploitation of the energy available inside thepreheating tunnel.

SUMMARY OF THE INVENTION

The general objective of the present invention is to overcome thedrawbacks of the known art and, in view of this objective, according tothe invention, to improve the exploitation of the energy present in thefumes for heating the charge.

More specifically, the objective of the present invention is to increasethe heat exchange between the hot process fumes and the metal charge.

The above objectives are achieved by a process and plant producedaccording to the enclosed independent claims and subordinate claims.

Thanks to the present invention, the heat exchange between the hotprocess fumes and the metal charge is improved by increasing theturbulence and the mixing of the gaseous stream inside the preheatingtunnel, with a consequent acceleration of the combustion processes andan increase in the convective heat exchange coefficients between saidcombustion gas and charge material. This effect is obtained withhigh-speed jets of gas; the gas used is generally air, but the use of adifferent gas is not excluded, if needed to manage the chemicalcomposition inside the preheating tunnel; this gas can also beadvantageously preheated.

In particular, an object of the present invention relates to a processand plant for preheating a metal charge fed in continuous to an electricmelting furnace through a preheating tunnel provided with a horizontalconveyor, wherein said metal charge is hit, in countercurrent, by theexhaust fumes or gases leaving said electric melting furnace and by jetsof gas ejected through a plurality of nozzles positioned in the hood ofsaid tunnel provided with side walls and said hood. Said nozzles arearranged in groups interspaced with respect to each other in alongitudinal direction with respect to the tunnel, and generate asmall-scale turbulence or inject small fast gas jets that can penetratethe stream, and said nozzles simultaneously generate a “horseshoevortex” structure, consisting of a descending central gas flow(“downwash”), and ascending flows (“upwash”) close to the side walls ofthe preheating tunnel which allow the necessary circulation of thegases.

According to the present invention, said nozzles are arranged in groups,in each of which the nozzles are aligned in correspondence with certaintransversal sections of the hood of the tunnel, suitably spaced apart.This allows a small-scale turbulence and simultaneously a large-scalevortex structure to be generated: the first one corresponds to the factthat the small fast jets of gas are able to penetrate the main gasstream passing through the tunnel, considerably accelerating the mixingand combustion of the gases; the large-scale vortex structure, whichincreases the heat exchange between fumes and charge, is commonlydefined a “horseshoe vortex” and is characterized by a descendingcentral flow (“downwash”), which increases the heat exchange in thecenter of the preheating tunnel, and ascending flows (“upwash”) close tothe side walls of the tunnel which allow the necessary circulation ofthe gases, and which, after transferring part of their heat energy tothe metal charge in the descending phase, limit the heat exchange withthe side walls of the tunnel and horizontal conveyor. Contrary to whatis disclosed in the known art, the above-mentioned gas jets are notarranged uniformly along the preheating tunnel but are rather arrangedin groups, at least two, suitably interspaced; this is to avoidinterference of a fluid-dynamic nature and to allow, first of all, agood mixing of the gases and a rapid development of the combustion (witha small-scale turbulence effect) and subsequently pushing them towardsthe metal charge (with the “horseshoe vortex” effect).

Contrary to what is present in the known art, the nozzles are notdimensioned so as to supply all the combustion air in a distributed anduniform mode, but, instead, they are dimensioned as small fast jetswhose primary function is to supply kinetic energy and modify the fieldof motion according to what is described above; for this reason, theabove-mentioned jets can be more accurately defined as “fluid-dynamicturbulence generators” or more simply, “fluid-dynamic turbulators”.

The use of “fluid-dynamic turbulators” is much easier and cheaper thanthe alternative solution of increasing the turbulence inside thepreheating tunnel by the insertion of deflector panels, or so-called“static turbulators”; these deflectors must operate within a gas flowcharacterized by high temperature and high content of dust and they aretherefore normally built as water-cooled metal panels, which is not anefficient solution from a thermal point of view; independently from howthese deflector panels are built, their use has been practicallyabandoned due to their rapid wear and frequent breakages.

The known art does not take into consideration the fact that inpractice, there is always a significant infiltration of ambient air intothe charge preheating tunnel through unavoidable openings, and that thequantity of combustion air is variable during the process, whereas theneed for a good mixing is a substantially constant.

The advantage deriving from the present invention is therefore evident,whereby the operation of jets for controlling the turbulence inside thepreheating tunnel is substantially decoupled by the control of thesupply of possible combustion air.

BRIEF DESCRIPTION OF THE DRAWINGS

The structural and functional characteristics of the invention and itsadvantages with respect to the known art will appear more evident fromthe following description, referring to the attached drawings, whichillustrate a possible non-limiting embodiment of the invention itselfapplied to an electric arc furnace (EAF) for melting metal scrap chargedin continuous.

In the drawings:

FIG. 1 illustrates a plant according to the known art;

FIG. 2 illustrates a plant according to the known art provided withcombustion air injectors uniformly distributed along the preheatingtunnel;

FIG. 3 illustrates a plant according to the known art provided with apreheating section with burners and a heating section by means ofoff-gas connected by a suction/evacuation section of the fumes;

FIGS. 4 a, 4 b and 4 c show, with a side view (4 a) and views from above(4 b and 4 c), the flow of gases in the preheating tunnel of a plantproduced according to the known art without high-speed air jets, inparticular the view from above in FIG. 4 c shows the absence ofturbulence in correspondence with air injectors produced and arrangedaccording to the known art;

FIG. 5 illustrates a plant according to the present invention;

FIG. 6 illustrates the arrangement of the air injectors in a preheatingtunnel of a plant according to the known art;

FIG. 7 illustrates the arrangement of the nozzles in a preheating tunnelof a plant according to the present invention;

FIG. 8 illustrates the arrangement of the nozzles in a preheating tunnelof a plant according to the present invention;

FIGS. 9 a and 9 b show the different flow of gases in a plant producedaccording to the present invention with the use of “fluid-dynamicturbulators” formed with high-speed jets;

FIG. 10 illustrates a side view of the preheating tunnel with thenozzles provided in each section according to the present invention;

FIG. 11 illustrates, in a cross-section, a portion of the preheatingtunnel with the nozzles provided according to the present invention;

FIG. 12 illustrates a group of nozzles, according to the presentinvention, that can be positioned in a section of the preheating tunnel.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to the figures, FIGS. 1-3 illustrate three plantsproduced according to the known art, in particular, FIG. 1 illustrates atraditional plant with a preheating tunnel without gas injectors; FIG. 2illustrates a plant with air injectors arranged in the preheating tunnelaccording to the known art; FIG. 3 illustrates a plant with a heatingand a preheating section, with burners, connected by asuction/evacuation section of the fumes.

In the figures, 1 indicates as a whole a plant for continuously feedinga metal charge of scrap 11 to an Electric Arc Furnace (EAF) 12 in whicha bath of molten metal is present in the liquid state.

In these configurations, the flow 17 of fumes coming from the furnace 12follows a substantially linear path, which tends to be aligned with thewalls of the preheating tunnel, thus moving away from the metal charge11. Also in the configuration of FIG. 2 , with the injectors ofcombustion air 19, the flow 17 does not undergo significant deviations,as the combustion air introduced by the injectors 10 is normallyextremely limited, due to the infiltrations of cold air 18 which arepresent at the interface between the furnace 12 and the tunnel and whichare almost always sufficient for completing the combustion of the fumes17 coming from the furnace 12.

A plant 1 of this type is described for example in U.S. Pat. No.5,183,143.

The plant 1 comprises at least one horizontal conveyor 13 suitable forcontinuously moving the metal charge of scrap 11 towards the electricmelting furnace EAF 12, defining the respective continuous horizontalfeeding line of the charge 11 to a charging area IV of the furnaceitself 12.

As can be clearly seen from the drawings, the horizontal conveyor 13forms the base of a preheating tunnel 16 of the metal charge of scrap11.

More specifically, the plant 1 is composed of a preheating section IIIwhich brings the metal charge of scrap 11 into the electric meltingfurnace EAF 12, of an evacuation section of the fumes II present in theplant 1, located, considering the movement direction of the scrap 11,upstream of said preheating section III, and of a section I whichreceives the metal charge of scrap 11 with a conventional receivingsystem of the scrap 11.

The horizontal conveyor 13 conveys the metal charge of scrap 11 byoscillation and transfers it from the preheating section III to amovable terminal section, called “connecting car”, which leads the scrap11 into the electric melting furnace EAF 12.

According to the present invention, nozzles are present on the hood ofthe tunnel of the preheating section III (preheating tunnel 16), for theinjection of gas 15.

In particular, these are nozzles for the high-speed injection of gas 15.

Said nozzles 15 are distributed so as to obtain a whirling turbulentmotion inside the preheating tunnel 16 to improve the heat exchangebetween the off-gases 17 and metal charge of scrap 11.

As illustrated in FIGS. 9 a and 9 b , the nozzles 15 provided on thehood of the tunnel 16 of the preheating section III increase theturbulence of the off-gases 17 thus allowing the following to beobtained:

a greater mixing velocity of the reacting gases and their combustion;

better conditions for completing the combustion of CO, H.sub.2 and othergases and possible carbonaceous particulate coming from the electricmelting furnace EAF 12;

an improved and more uniform temperature distribution inside thepreheating tunnel 16,

a better heat exchange between the combustion gases and metal charge 11on the horizontal conveyor 13 inside the preheating tunnel 16.

In the plants of the known art devoid of said nozzles or “fluid-dynamicturbulators” 15, the air entering the plant through the connectionportions is uncontrolled, and with a limited turbulence and vorticity,without adequately mixing with the gases (FIGS. 4 a, 4 b and 4 c ), andtherefore causing a slow and often incomplete combustion inside thepreheating tunnel 16.

Thanks to the presence of the “fluid-dynamic turbulators” 15 inside thepreheating tunnel 16, on the contrary, a greater mixing of the gases isobtained together with a higher flame intensity, which also help tolimit the cooling due to the entry of air from outside of the plant, inparticular into the preheating tunnel 16.

As illustrated in FIGS. 7 and 9 a, 9 b, the arrangement of the nozzlesor “fluid-dynamic turbulators” 15 allows the so-called downwash portionof the field of motion to be concentrated on the central portion of thehorizontal conveyor 13, where the maximum heat exchange is thereforeobtained between the metal charge 11 and the gases/fumes 17 present inthe preheating tunnel 16.

In order to obtain the above-mentioned whirling-motion configuration,the nozzles 15, and therefore the jets, are distributed transversely onthe hood of the preheating tunnel 16 in a non-uniform manner with agreater concentration on the top of the hood of the tunnel 16.

The arrangement of the gas jets is therefore such as to obtain awell-defined whirling structure (FIGS. 9 a, 9 b ) inside the preheatingtunnel 16, characterized by:

a descending-flow area in the central area, immediately downstream ofthe section in which the nozzles 15 are present, in order to increasethe heat exchange with the metal charge of scrap 11 in this area,

ascending areas on the sides, to limit the heat exchange with the wallsof the preheating tunnel 16 and horizontal conveyor 13.

This whirling structure of gases in the preheating tunnel 16 is commonlycalled a “horseshoe vortex” and is obtained, according to an embodimentof the present invention, by arranging the nozzles 15, and therefore thejets, over about ⅔ of the cross section of the preheating tunnel 16,leaving the two side walls, close to the side walls of the hood of saidtunnel 16, free.

The arrangement of the nozzles on the hood of the preheating tunnel 16can vary in relation to the specific plant issues (see for example theembodiment solution shown in FIG. 8 ), maintaining anyway therequirement that the high-speed jets always intercept the centralportion of the flow of off-gases 17, leaving the side portions free andthus favoring the establishment of an upward circulation of the gasesand the formation of a horseshoe whirling motion in the entrainmentflow.

The high-speed jets that act as “fluid-dynamic turbulators” are notuniformly distributed longitudinally on the hood of the preheatingtunnel 16, but according to “injection sections” adequately spaced apartfrom each other, so as to avoid fluid-dynamic interference phenomena;the distance between two adjacent injection sections should be 4-6meters depending on the velocity of the gases passing through thepreheating tunnel. The space between two adjacent injection sections isneeded to allow the high-intensity flame produced by the sectionupstream to have time to develop before being pushed towards the chargeby the injection section immediately downstream.

In order to best exploit the length of the preheating tunnel 16 forimproving the heat recovery and completing the combustion of the CO andH.sub.2 and possible pollutants in the process gases, the firstinjection section is positioned as close as possible to the electricmelting furnace EAF 12.

The first group of fast gas jets is located in proximity of the electricmelting furnace 12, within a distance of 7-10 meters from the same.

The invention provides for gas jets with velocities and/or flow-ratesthat increase among subsequent “gas injection sections”. The number ofinjection sections varies from two to four, depending on the quantity ofcombustible gas produced by the melting process under consideration.

As illustrated in FIG. 11 , starting from the electric melting furnaceEAF 12 and following the flow of gases towards the fume suction plant(gas flow opposite to the movement direction of the metal charge 11),the plurality of nozzles 15 forming the first injection section can belocated above the connecting car (first water-cooled hood) whereas thenozzles 15 forming the other injection sections can be arranged at thebeginning of each segment of the refractory section of the preheatingtunnel 16 (refractory lined hood).

In the example, three injection sections are used, each composed of fournozzles 15.

The gas injected is generally air but another gas can also be adoptedand the gas used can also be preheated.

Control means of the operating conditions of the nozzles 15 can beprovided in each section.

The jets released from the nozzles 15 are small and fast as they must becapable of providing both a mixing and deviation action on the stream ofgas passing through the preheating tunnel 16, enabling the “downwash”motion of the hot gases 17 towards the metal charge 11, with a velocitysufficient for penetrating the interstices of the material (theso-called “impingement” effect, as visible in FIG. 5 , where the flow ofhot gases 17 is pushed downwards towards the metal charge 11), thusimproving the convective heat exchange.

This effect is obtained by assessing the flow-rate conditions andvelocity of the gas flows involved in the process in question: definingimpulse of a fluid stream as the product between mass flow-rate andvelocity of the stream itself, each single jet shall be designed in sucha way that the set of jets has an impulse similar to the impulse of themain stream of fumes passing along the tunnel 16 from the furnace 12 tosuction plant.

The design condition is therefore the following:

$\frac{W_{gas} \cdot V_{gas}}{N_{jets} \cdot \left( {W_{jet} \cdot V_{jet}} \right)} \approx 1$wherein:W_(gas)=mass flow-rate of the fumes in the tunnel (16) in a giveninjection section [kg/s]W_(jet)=mass flow-rate of the single jet in the same injection section[kg/s]V_(gas)=velocity of the fumes in the tunnel in correspondence with thesame injection section [m/s]V_(jet)=velocity of the single jet [m/s]N_(jets)=number of jets on the given injection section [#]

For purely illustrative purposes, in the application described,considering the injection of air at room temperature, this condition isnormally obtained with jets having a flow-rate of around 1,000Nm.sup.3/h and a discharge velocity ranging from 85 to 105 m/s.

Within the proposed technical solution, due to the gas injected, aprogressive increase in the flow-rate of gas passing through thepreheating tunnel 16, is produced, therefore it may be necessary toconsider jets with a greater impulse for the injection sectionspositioned further away from the furnace 12.

Following the flow of gases leaving the melting furnace 12 and enteringthe preheating tunnel 16, the first injection sections are designed touse lower flow-rates and velocities than the subsequent injectionsections, due to the overall increasing flow-rate of the gas passingthrough the preheating tunnel.

Each injection section can be managed, controlled and regulatedindependently of the others, depending on the process status and thecharacteristics of the charge 11 present in the preheating tunnel 16 andgases 17 leaving the furnace.

In the simplest embodiment, the nozzles 15 are all the same and arrangedon the top of the preheating tunnel 16 and their number basicallydepends on the width of the preheating tunnel 16 itself, considering anavailability of about ⅔ of the central portion (where the “downwash”area is to be established) with a distance between each jet of about450-500 mm. In order to obtain an effective impingement effect, the topof the preheating tunnel 16 must be positioned at a distance of around800-1,200 mm from the charge present in the conveyor (if the presentinvention is applied to an existing plant, this may require aredesigning of the preheating tunnel). In case of particularconfigurations of the preheating tunnel 16, for example in the presenceof plant constraints that do not allow the nozzles 15 to remain at thesame distance from each other, a different arrangement and dimensioningof the jets can be used for obtaining an equivalent fluid-dynamiceffect.

Contrary to the known plants and processes wherein the air injection islinked to the control of the combustion process from a stoichiometricpoint of view, in the present invention the jets of air or other gas aremainly used for obtaining certain turbulence conditions inside thepreheating tunnel 16.

Even in the most common case of the use of air jets, the overallinjection capacity of the system described is almost always lower thanthe flow-rate of air necessary for completing the combustion of theprocess gases 17 coming from the furnace 12, as the primary objective ofthe system described is to stabilize the turbulence; the control of thesupply of combustion air of the process gases inside the preheatingtunnel 16 is basically delegated to the modulation of the suctiondepression and width of the gap between the entrance of the preheatingtunnel and the furnace (that can never be completely eliminated from aplant-engineering point of view). In this way, the injection of air asturbulence generator is significantly decoupled by the supply of ambientair for completing the combustion of the process gases.

The final objective of the present invention is to increase thecombustion intensity of the process gases coming from the furnace andthe heat exchange between them and the charge, thus increasing theoverall energy efficiency of the melting process.

The improved mixing and combustion of the process gases released fromthe furnace achieved with the present invention allows to obtain abetter thermal destruction of the polluting substances (and relatedprecursors) present therein.

The present invention can also be applied to plants such as thosedescribed, for example, in the document WO2012007105 provided with apreheating tunnel and a heating tunnel of the metal charge of scrap.

Thanks to the present invention, the scrap charge heating takes placethrough a turbulence created in the preheating tunnel 16, unlike whathappens in the known plants in which the introduction of air is carriedout according only to the requirements of the chemical combustionprocess, without any link to the control of the motion field inside thepreheating tunnel.

Thanks to the present invention, the use of deflectors inside thepreheating tunnel 16 can also be avoided. These deflectors havedisadvantages as they cause a significant pressure drop in the suctionof the fumes, they require cooling and frequent maintenance operationsas they operate within a flow of very hot and dusty gases (this not onlyrepresents a plant complication and a potential risk of leakages, butalso causes a useless loss of thermal energy by the gases), they aredifficult to regulate and manage from a practical point of view as it isnot easy to change the incidence angle, and finally, their effect islimited when the velocity and therefore flow-rates to be treated arelow.

The objectives of the invention mentioned in the preamble of thedescription have therefore been achieved.

The protection scope of the present invention is defined by the enclosedclaims.

The invention claimed is:
 1. A process for preheating a metal charge fedin continuous to an electric melting furnace through a preheating tunnelprovided with a horizontal conveyor, the process comprising: causingsaid metal charge to be hit, in countercurrent, by exhaust fumes or afirst gas leaving said electric melting furnace, and by jets of a secondgas ejected through a plurality of nozzles positioned on a hood of saidpreheating tunnel provided with side walls and said hood, wherein theprocess further comprises, arranging said plurality of nozzles in groupsinterspaced from each other in a longitudinal direction with respect tothe preheating tunnel, and generating a turbulence by injecting thesecond gas to penetrate a stream of the first gas passing through thepreheating tunnel, said nozzles simultaneously generating a vortexstructure that comprises a descending central gas flow, and ascendingflows adjacent to the side walls of the preheating tunnel, so as toenable a desired circulation of the first and the second gases, whereina single jet of a nozzle of the plurality of nozzles is dimensioned sothat a set of the jets has an impulse equivalent to an impulse of a mainstream of the fumes that are passing along the preheating tunnelaccording to the following dimensioning condition:$\frac{W_{gas} \cdot V_{gas}}{N_{jets} \cdot \left( {W_{jet} \cdot V_{jet}} \right)} = 1$wherein: W_(gas)=mass flow-rate of the fumes in the preheating tunnel ina given injection section [kg/s], W_(jet)=mass flow-rate of the singlejet in the given injection section [kg/s], V_(gas)=velocity of the fumesin the preheating tunnel in correspondence with the given injectionsection [m/s], V_(jet)=velocity of the single jet [m/s], N_(jets)=numberof the jets on the given injection section.
 2. The process according toclaim 1, wherein the jets released from said plurality of nozzles aredistributed transversely on the hood of the preheating tunnel in anon-uniform manner, with a greater concentration on a top of the hood ofsaid preheating tunnel.
 3. The process according to claim 1, wherein thejets released from said plurality of nozzles are distributed over about⅔ of a cross-section of the preheating tunnel, leaving at least part ofthe side walls of the hood of said preheating tunnel free.
 4. Theprocess according to claim 1, wherein the nozzles are non-uniformlydistributed longitudinally along said preheating tunnel, whereby saidplurality of nozzles are provided at gas injection sections that arespaced apart with longitudinal sections of the preheating tunnel withoutsaid plurality of nozzles in order to avoid interference phenomena. 5.The process according to claim 4, wherein a first group of the nozzlesis provided closer to the electric melting furnace than a remainder ofthe nozzles, and within a distance of 7-10 meters from the electricmelting furnace.
 6. The process according to claim 4, wherein theplurality of nozzles provide jets having velocities or flow rates thatincrease among subsequent gas injection sections.
 7. A plant forimplementing a process for preheating a metal charge fed in continuousto an electric melting furnace, comprising: a preheating tunnel providedwith a horizontal conveyor, wherein said metal charge is hit, incountercurrent, by a exhaust fumes or a first gas leaving said electricmelting furnace; and a plurality of nozzles positioned on a hood of saidpreheating tunnel provided with side walls and said hood, said pluralityof nozzles emitting jets of a second gas further hitting said metalcharge, wherein at least two groups of the plurality of nozzles areprovided on the hood of said preheating tunnel, interspaced from eachother in a longitudinal direction with respect to the preheating tunnel,wherein the plurality of nozzles generate a turbulence by injecting thejets of the second gas to penetrate a stream of the first gas streampassing through the preheating tunnel, said nozzles simultaneouslygenerating a horseshoe vortex structure comprising a descending centralgas flow that forms a descending flow, and ascending flows that forms anascending flow adjacent to the side walls of the preheating tunnel, soas to enable a desired circulation of the first and the second gases,and wherein a single jet of a nozzle of the plurality of nozzles isdimensioned so that a set of the jets has an impulse equivalent to animpulse of a main stream of the fumes that are passing along thepreheating tunnel according to the following dimensioning condition:$\frac{W_{gas} \cdot V_{gas}}{N_{jets} \cdot \left( {W_{jet} \cdot V_{jet}} \right)} = 1$wherein: W_(gas)=mass flow-rate of the fumes in the preheating tunnel ina given injection section [kg/s], W_(jet)=mass flow-rate of the singlejet in the given injection section [kg/s], V_(gas)=velocity of the fumesin the preheating tunnel in correspondence with the given injectionsection [m/s], V_(jet)=velocity of the single jet [m/s], N_(jets)=numberof the jets on the given injection section.
 8. The plant according toclaim 7, wherein said plurality of nozzles are distributed transverselyand non-uniformly on the hood of the preheating tunnel, with a greaterconcentration on a top of the hood of said preheating tunnel.
 9. Theplant according to claim 7, wherein said nozzles are distributed overabout ⅔ of a cross-section of the preheating tunnel, leaving at leastpart of the side walls of the hood of said preheating tunnel free. 10.The plant according to claim 7, wherein said plurality of nozzles aredistributed longitudinally and non-uniformly on the hood of thepreheating tunnel, and wherein said plurality of nozzles are provided atsections along said preheating tunnel that are gas injection sectionsspaced apart with longitudinal sections of the preheating tunnel withoutsaid plurality of nozzles in order to avoid interference phenomena. 11.The plant according to claim 10, wherein there are from 2 to 4 of saidgas injection sections spaced about 4-6 meters from each other in alongitudinal direction with respect to the tunnel.
 12. The plantaccording to claim 10, wherein a first gas injection section of the gasinjection sections is positioned closer to the electric melting furnacethan a remainder of the gas injection sections and within a distance of7-10 meters from the electric melting furnace.
 13. The plant accordingto claim 10, wherein three of the gas injection sections are providedalong the preheating tunnel, with four nozzles per gas injectionsection.
 14. The plant according to claim 10, wherein the plurality ofnozzles provide jets having velocities or flow rates that increase amongsubsequent gas injection sections.
 15. The plant according to claim 7,wherein the plurality of nozzles are arranged on a top of the preheatingtunnel in a number depending on a width of the preheating tunnel, with adistance of about 450-500 mm between each jet.